Underwater surveillance systems for determining the direction from which acoustic signals arrive at a listening station unambiguously, are well known. Ambiguities are resolved by resorting to special types of array configurations and/or processing. Two examples of such systems are described in U.S. Pat. Nos. 3,870,989 issued to Alfred L. Mallet on Mar. 11, 1975, and 3,987,404 issued to Thomas E. Woodruff on Oct. 19, 1976. Each of these patents describe a sonobuoy utilizing two bi-directional transducers, an omni-directional transducer and a compass. Signals from these components are processed by additional equipment, and the direction from which the acoustic signals are arriving is determined.
The single sonobuoys described by the above-mentioned patents are eminently satisfactory for the relatively short range operation for which they are intended. However, there is also a need for a different kind of system, one having longer range, a greater sensitivity, and a high signal-to-noise ratio and a sharper directivity. Such a system could be obtained by an array of many elements exhibiting a response pattern comprising a single, narrow beam, preferably steerable at least in one plane. Usually, this plane is the horizontal plane since signals from distant targets essentially arrive in the horizontal plane.
One approach to the problem would be to construct a two-dimensional, or even a three-dimensional array of transducers, each omni-directional, bi-directional, or uni-directional, together with suitable beam-forming equipment. However, when the frequency of the sounds of interest and their wavelength in water is considered, the difficulties in constructing and deploying arrays of sufficient size make this approach to the problem unattractive.
In another approach, a "line" array of "omni-directional" transducers could be utilized with beam forming equipment to provide a response pattern having a set of narrow lobes with the narrow lobes being formed in the phasing and summation process utilized in the beam forming.
In this type system the beam is normally a cone-shaped beam, not only with a main lobe formed in one direction, but an "image" lobe is also formed in a different direction, such that it is impossible to tell from which direction an incoming signal is received. Thus, when utilizing a string of omni-directional elements, there is an ambiguity in determining the direction of the incoming signals.
It is possible with properly aligned "bi-directional" elements to sum their outputs so as to provide beam steering. However, the result of the above process nonetheless results in a main lobe and an image lobe. Thus, directional ambiguities occur.